Most power semiconductor devices, including devices such as power MOSFETs and insulated gate bipolar transistors (IGBTs), typically have been fabricated with silicon (Si) semiconductor material. More recently, silicon carbide (SiC) power devices have been considered due to their superior properties. III-N semiconductor devices, such as gallium nitride (GaN) devices are now emerging as attractive candidates to carry large currents, support high voltages and to provide very low on-resistance and fast switching times.
Typical GaN high electron mobility transistors (HEMTs) and related devices are normally on, which means that they conduct current at zero gate voltage. These typical devices are known as depletion mode (D-mode) devices. However, it is more desirable in power electronics to have normally off devices—called enhancement mode (E-mode) devices—that do not conduct current at zero gate voltage and thus avoid damage to the device or to other circuit components by preventing accidental turn on of the device.
FIG. 1 shows a prior art Ga-face GaN HEMT depletion mode structure. Substrate 10 may be GaN, SiC, sapphire, Si, or any other suitable substrate upon which a GaN device may be formed. GaN buffer layer 14 and AlxGaN layer 18 on top of it are oriented in the [0 0 0 1] (C-plane) direction. The conducting channel consists of a two-dimensional electron gas (2DEG) region, shown by a dotted line in GaN buffer layer 14 in FIG. 1, is formed in layer 14 near the interface between layer 14 and AlxGaN layer 18. A thin, 0.6 nm AlN layer (not shown) is optionally included between GaN layer 14 and AlxGaN layer 18 in order to increase the charge density and mobility in the 2DEG region. The region of layer 14 between the source 27 and the gate 26 is referred to as the source access region. The region of layer 14 between the drain 28 and gate 26 is referred to as the drain access region. The source 27 and drain 28 both make contact with buffer layer 14. With no applied gate voltage, the 2DEG region extends all the way from the source 27 to the drain 28, forming a conducting channel and rendering the device normally on, making it a depletion mode device. A negative voltage must be applied to the gate 26 to deplete the 2DEG region under the gate 26, and thus to turn the device OFF.
Another related prior art III-N HEMT device is the subject of provisional application Ser. No. 60/972,481, filed Sep. 14, 2007, entitled “III-N Devices with Recessed Gates,” which application is hereby incorporated by reference herein.